The overall objectives of the proposed work are to exploit the process of skeletal myogenesis and the techniques of somatic cell hybridization in an effort to explore nucleo-cytoplasmic interactions which control phenotypic expression during terminal cell differentiation. The cells to be utilized include chicken embryo and rat fetus skeletal muscle myoblasts and myogenic cell lines of rat and mouse origin. Cell hybrids, heterokaryons, cytoplasmic hybrids and reconstituted cells will be produced by polyethylene glycol-mediated somatic cell fusion, and the fusion products identified both by selective and morphological markers. Myogenic capabilities will be assessed by means of such criteria as multinucleated muscle fiber formation, increases in enzyme activities characteristic of skeletal muscle, and by immunofluorescent staining with antibodies directed against skeletal muscle proteins.